Offgases from thermal processes (=combustion gases) are the no longer usable gaseous and/or powderlike waste products arising during a combustion. Depending on the fuel, the offgases contain various portions of harmful components to man and the environment, such as sulfur oxides (SOx), hydrogen chloride (HCl), nitrogen oxides (NOx) and/or dioxins. Consequently, an offgas cleaning is usually required for technical and industrial facilities. One generally distinguishes between dry and wet offgas cleaning methods.
In dry offgas cleaning methods the offgases are brought into contact with solids which react with the harmful components of the offgases. In this way, the harmful components are removed from the offgas. For example, one uses for desulphurization quicklime (CaO), hydrated lime (Ca(OH)2) and/or limestone meal (CaCO3). Limestone refers to sedimentary rocks which consist for the most part of calcium carbonate (CaCO3) in the form of the minerals calcite and aragonite. Limestone meal is pulverized limestone. Quicklime is made by burning of limestone. Quicklime consists of calcium oxide (CaO), apart from slight impurities. Hydrated lime or also slaked lime arises under intense development of heat when quicklime is reacted with water. This is also called slaking. Hydrated lime is a bright powder or meal and consists of calcium hydroxide (Ca(OH)2), apart from slight impurities. All these calcium compounds react with the SO2 and SO3 contained in the offgases to form essentially calcium sulfite (CaSO3) or calcium sulfate (CaSO4). The calcium sulfate is then separated in corresponding filters. It is basically known to blow powderlike or meal-like desulphurization agents, especially limestone meal, into the combustion chamber or the offgas flow.
Alternatively, the reaction occurs in a solids/offgas reactor downstream from the combustion, such as a packed bed filter, in which the offgas being cleaned flows through a loose granular layer of the sorption agent. The packed bed filter can be designed, e.g., as a solid bed absorber (with stationary fill), migrating bed absorber (with moving fill) or fluidized bed absorber (with formation of a fluidized bed). Packed bed filters are usually not operated with powderlike sorption agents, but instead with granular sorption materials, on account of excessively high pressure loss.
Buildup-agglomerated sorption granules made entirely of hydrated lime and/or limestone meal are known. Agglomeration is an umbrella term for methods of mechanical grain enlargement and it means the fusing and binding together of finely dispersed solid primary particles to form larger particle associations, the agglomerates. A distinction is drawn between pressing agglomeration and buildup agglomeration. In pressing agglomeration, external forces are exerted on the particles of a dry bed by pressing dies, so that many points of contact are formed with short contact distances. In buildup agglomeration, the individual primary particles or already formed small agglomerates are fused to each other by relative movement and wetting, optionally with an additional binding agent. Thanks to this relative movement, an interaction between binding forces and separating forces occurs. The separating forces arise from impact, shear, falling and friction stresses. Agglomeration occurs when the binding forces outweigh the others. One distinguishes four kinds of buildup agglomeration, namely, roll granulation, mix granulation, fluid bed granulation and granulation in liquids.
In buildup granulation with moisture, the individual primary particles, especially the hydrated lime granules, join together tightly with the moisture. This produces a very dense, nearly closed framework. As a result, the molecules of harmful gas can only penetrate with difficulty into the interior of the known sorption granules. In particular, it has been found that SOx are only absorbed in shell-like manner, in a relatively thin outer shell region, forming calcium sulfate (CaSO4). The Ca(OH)2 remains unused by up to 40 wt. %, for example, inside the known granules. Apparently the diffusion of the SOx gas is further impeded by the formation of calcium sulfate in the outer shell region.
Another mineral gas absorption granular material is known from DE 10 2009 045 278 A1. This comprises calcium-based porous granules, having a core containing at least 80 wt. % of calcium carbonate (CaCO3) and at least one agglomeration layer enclosing the core and containing calcium hydroxide (Ca(OH)2). The granules have a portion of calcium hydroxide of at least 60 wt. % in relation to the entire dry mass of the granules, as well as an essentially spherical shape and a BET surface of at least 8 m2/g. The granules according to one preferred embodiment should have micropores with pore diameters of less than 100 μm, mesopores with pore diameters of 100 to 500 μm and/or macropores with pore diameters above 500 μm. The mineral desulphurization agent of DE 10 2009 045 278 A1 is produced by means of buildup agglomeration by placing a mixed product comprising at least calcium hydroxide in the form of powder and a mother grain containing at least 80 wt. % of calcium carbonate, and water in a granulation or pelletizing mixer, producing granules by granulation, and drying the granules so produced.
In the context of the present invention, however, it has been discovered that the high water content and the porosity of the granules known from DE 10 2009 045 278 A1 likewise do not produce the expected high SOx separation performance. As in the case of the hydrated lime granules without “mother grain”, an outer shell absorption region is formed, hindering the separation process, while relatively much unconsumed or unused hydrated lime is still present inside the granules. Apparently the water adsorbed at room temperature is not available in the granules to promote a reaction at higher temperatures where the absorption processes and offgas cleaning take place. These high temperatures are known to be generated by offgas temperatures between, for example, 100 and 900° C. There also appears to occur here a kind of clogging due to formation of calcium sulfate in the outer peripheral regions of the granules, in addition to the formation of a diffusion-tight shell due to the production technology, so that the further penetration of gas into the granules is hindered.
In order to prevent this, it is proposed in DE 10 2011 112 657 A1 to use buildup-agglomerated granules having, besides hydrated lime and/or limestone meal, a capillary water storing light aggregate meal and an adsorbed water content between 2 and 30 wt. % in terms of the total of the components of the calcium compound, the light aggregate, and the water. Light aggregates possess the ability to store capillary water such that water is available in sufficient amount in the granules even at higher temperatures, such as occur for example in offgas cleaning plants downstream from combustion layouts. This is supposed to favor the reaction between the SOx gases and the hydrated lime meal grains and/or the limestone meal grains in that water is available on the surface of the meal grains for the dissolving of SOx gases and the formation of calcium sulfate can occur from the solution. In any case, the water storing light aggregate brings about a boosting of the degree of separation of the SOx gases. Furthermore, it has the effect that almost the entire Ca(OH)2 or the entire CaCO3 of a granule can react with the SOx gas to form calcium sulfate, without the outer reaction layers blocking the progress of the reaction. A “clogging” such as occurs with the known granules of similar makeup but without light aggregate is largely prevented according to DE 10 2011 112 657 A1.
A granular sorption material is known from DE 10 2011 113 034 A1, whose granules are shaped bodies made in a pressing process. The granules have hydrated lime and/or limestone meal as the SOx absorption agent and in addition a finely divided, shredded cellulose fiber material. Furthermore, the granules have an adsorbed water content of up to 30 wt. % in terms of the quantity of SOx absorption agent and shredded material. The shredded cellulose fiber material increases the porosity or capillarity of the granules, which improves the diffusion of the harmful gases into the granules. Furthermore, a water reservoir is also created, thereby increasing the separation performance.
The granular sorption materials known from DE 10 2011 112 657 A1 and DE 10 2011 113 034 A1 have proven to work well.